This invention relates to a rigid thermoformable packaging material and containers made therefrom. More specifically, it relates to a composite material having a layer of oriented polystyrene laminated to a functional polymer resin layer which serves as a barrier to moisture, odors, gases, chemicals and the like and/or improves the toughness and resistance to cracking and tearing of the material.
A wide variety of polymer resins are presently used to package medical and dental products as well as foods. These polymers are formed into sheets, films, and containers and include polystyrene, polyolefins, polycarbonate, acrylonitriles, polyvinyl chloride, polyethylene terephthalate polyesters, and copolymers such as styrene-butadiene and acrylonitrile-butadiene-styrene-polymethylmethacrylate. All of these resins have certain properties which make them desirable for these end uses.
However, when used as monolayer packaging materials, each group of resins has certain drawbacks. For example, while polyolefins have good moisture barrier properties, they are difficult to thermoform. Polystyrene, on the other hand, is readily thermoformable but has poor barrier properties. Poor resistance to some chemicals, and tends to be brittle.
It is known to laminate a plurality of layers of dissimilar polymer resins in an attempt to obtain materials having a combination of the desirable properties of the individual resins. For example, composite multilayer films of polystyrene and polyolefins have been made in attempts to take advantage of the stiffness and rigidity of polystyrene and strength, durability and barrier properties of the polyolefin, Erb. U.S. Pat. No. 3,589,976, teaches a composite film of a coextruded polystyrene core layer and polyolefin surface layers utilized as packaging materials for foods. Likewise, Japanese published applications Nos. 53-034,881; 55-144,158; and 55-163.164 generally teach composite films of polystyrene and polyolefins used as packaging materials.
However, for materials designed for use in packaging medical or dental products, or for certain food applications, the polymer resin or laminate structure must possess a multiplicity of attributes. Not only must the packaging material be rigid, but it also must resist cracking and tearing brought about by clamping equipment used on commercial thermoforming, filling, and sealing systems. Additionally, the material must be heat sealable and yet have clean peel characteristics upon opening. Finally, the packaging material desirably has good light transmission characteristics is resistant to chemical attack and is stable to radiation sterilization procedures. Heretofore, none of the commonly utilized packaging materials used in the art have possessed these desirable combinations of properties.
Accordingly, the need still exists in the art for a cost effective packaging material possessing all of the desirable attributes set forth above for use in the packaging of medical or dental products as well as certain foods.